1. Field of the Invention
This invention relates to metallic components for a ball grid array electronic package. More particularly, the bending of that component is reduced minimizing solder ball fracture due to fatigue.
2. Description of Related Art
Ball grid array electronic packages are used to house one or more integrated circuit devices, such as silicon based semiconductor chips. The packages have a base component with a plurality of metallized circuit traces. A semiconductor device is bonded to a central portion of the base component and electrically interconnected to inner ends of the circuit traces. The opposing outer ends of the circuit traces form an array. Solder balls are bonded to the points of the array. Those solder balls also bond the package to an external structure, such as a printed circuit board. A cover encapsulates the central portion of the base component, the semiconductor device and the inner ends of the circuit traces.
Typically, the base component is formed from plastic, as disclosed in U.S. Pat. No. 5,241,133 to Mullen, III et al., ceramic, or metal as disclosed in U.S. patent application Ser. No. 08/277,387 by Mahulikar et al. entitled "Integrally Bumped Electronic Package Components" that was filed on Jul. 19, 1994. Both U.S. Pat. No. 5,241,133 and U.S. patent application Ser. No. 08/277,387 are incorporated by reference in their entireties herein.
If the coefficient of thermal expansion of the electronic package base and of the external structure is different, fatigue related fracture of the solder balls may occur. Heating of the various components as a result of the semiconductor device generating heat during operation causes differential expansion of the various components to occur, resulting in stress on the solder balls. Cyclical repetition of this stress propagates cracks through the solder ball. When a crack extends across the diameter of a solder ball, the package fails.
Solder ball fatigue has been studied in plastic and ceramic base components for ball grid array packages. In a ceramic package, the stresses applied to the solder ball are parallel to the bonding plane of the base component and the solder balls in the row closest to the outer perimeter of the base fracture first. The increased distance from the center of the package to this outer row enhances the stress applied to the solder balls by thermally inducing movement of the package base.
One mechanism to reduce solder ball fatigue in a ceramic base component is disclosed in U.S. Pat. No. 4,581,680 to Garner. The Garner patent discloses that by increasing the height of the solder balls in the outer rows, the elastic limit of the solder joint is increased.
The Garner solution is inapplicable to metallic base components of ball grid array packages. With a metallic base, solder ball fracture is expected to occur in solder balls in the innermost row.
In a plastic package, the row of solder balls underlying the perimeter of the semiconductor device typically fail first.
There exists, therefore, a need to develop metal ball grid array package components that resist solder ball fatigue while retaining the thermal conductivity advantage of metal.